Aurivillius phase-based ceramics have been developed and patented by Symetrix Corporation for thin-film electronic applications. Such Aurivillius phases contain bismuth, together with at least one of strontium, calcuim, barium, and lead and with at least one of titanium, tantalum, and niobium; see, e.g., K. Kato et al, Journal of Materials Science: Materials in Electronics, Vol. 9, pp. 457-464 (1998). These Aurivillius phases give rise to ceramic materials such as strontium bismuth tantalate, strontium bismuth tantalate niobate, bismuth titanate, and strontium bismuth titanate.
Due to the fatigue-free properties of the ferroelectric material strontium bismuth tantalate (SBT), strontium bismuth niobate (SBN), and strontium bismuth tantalate niobate (SBTN), there has been considerable interest in fabricating new electronic devices from these materials. To date, these materials have predominately been made using a spin-on metal-organic decomposition (MOD) or sol-gel techniques. Other methods of depositing the films, such as sputtering, metal-organic chemical vapor deposition (MOCVD), liquid misted source chemical deposition (LMSCD), and jet or laser ablation deposition have been developed, but spin-on deposition remains the most popular deposition method. The reason for this is a combination of the simplicity of the spin-on process with the lack of major capitol equipment investment while still allowing the manufacture of very high quality films.
However, one big disadvantage of spin-on techniques is that they create large quantities of hazardous and environmentally hostile chemicals. One reason for this is because only a small fraction of the material initially placed on the wafer remains at the completion of the process. The remainder of the solution, including all of the hazardous solvents, are added only to facilitate processing and are removed from the film during the process as waste. In addition, large quantities of solvents are required for washing, rinsing and cleaning of the wafer surface prior to, during and after the spin-on deposition process.
As time progresses, the chemical process industry has become much more sensitive to the issue of hazardous wastes, such as the solvents used in these deposition processes, and has begun to act by regulating them or eliminating them. This has created a market for more environmentally-friendly materials and methods of manufacture. The semiconductor industry is well-known as a user of a large amount of hazardous materials and because of this has been targeted by communities and environmental groups. This has given the semiconductor industry a strong motivation to pursue more environmentally-friendly processes.
Thin film electrical devices using ferroelectric, paraelectric, and pryoelectric materials are an emerging technology with applications as diverse as nonvolatile memory, microactuators, gas sensors, and many others. However, the use of non-toxic environmentally benign materials for the processing of semiconductor devices (and virtually all processes/devices) is of utmost importance, due to the above-noted environmental concerns.
Symetrix Corp. has pioneered the work in SBT (SrBi.sub.2 Ta.sub.2 O.sub.9) thin films. See, e.g., C. A. Paz de Araujo et al, "Fatigue Free Ferroelectric Capacitors with Platinum Electrodes", Nature, Vol. 374, pp. 627-629 (April 1995); U.S. Pat. No. 5,434,102, "Process for Fabricating Layered Superlattice Materials and Making Electronic Devices Including Same", issued on Jul. 18, 1995, to H. Watanabe et al; U.S. Pat. No. 5,439,845, "Process for Fabricating Layered Superlattice Materials and Making Electronic Devices Using Same", issued on Aug. 8, 1995, to H. Watanabe et al; U.S. Pat. No. 5,514,822, "Precursors and Processes for Making Metal Oxides", issued on May 7, 1996, to M. C. Scott et al; and U.S. Pat. No. 5,784,310, "Low Imprint Ferroelectric Material for Long Retention Memory and Method of Making the Same", issued on Jul. 21, 1998, to J. D. Cuchiaro et al.
Raytheon Corporation (and its earlier predecessor Hughes Electronics) has continued to improve and innovate on this work, specifically involving SBTN (SrBi.sub.2 Ta.sub.2-x Nb.sub.x O.sub.9, where x=0 to 2). See, e.g., U.S. Pat. No. 5,721,009, "Controlled Carbon Content MOD Precursor Materials Using Organic Acid Anhydride", which describes the first use of an organic acid anhydride for fast controlled synthesis of MOD precursors for ceramic thin films; U.S. Pat. No. 5,885,648, issued Mar. 23, 1999, "Process for Making Stoichiometric Mixed Metal Oxide Films", describes the optimization of the SBTN materials for, e.g., ferroelectric applications; and U.S. Pat. No. 5,908,658, issued Jun. 1, 1999, "Process for Forming Thin Film Metal Oxide Materials Having Improved Electrical Properties", describes low leakage SBTN thin films developed for military applications.
Related application Ser. No. 09/307,589, filed May 7, 1999, "Polyether Acid Anhydride Useful for Improved Non-Toxic Solvent Soluble Group IV and V Metal Acid Salt Complexes" describes the first synthesis and use of the anhydride of 3,6-dioxaheptanoic acid for fast controlled synthesis of non-toxic solvent soluble MOD precursors for ceramic thin films. The titanium salt of 3,6-dioxaheptanoic acid is specifically described.
Another related application, application Ser. No. 09/395,695, filed concurrently herewith [PD-98128], describes the precursor and process for making "Environmentally Benign Group II and Group IV or V Spin-On Precursor Materials". The related application is directed to the formation of II-IV or II-V ceramic oxide thin films, such as barium strontium titanate (BST), which are prevalent materials used for DRAM, energy storage and other paraelectric and ferroelectric applications.
A review of the need for environmentally benign ceramic precursors and the alumina precursor made from 3,6-dioxaheptanoic acid and an alumina mineral (Group III precursor) is described in "Aqueous Synthesis of Water-Soluble Alumoxanes: Environmentally Benign Precursors to Alumina and Alumina Based Ceramics", Chemical Materials, Vol. 9, No. 11, pp. 2418-2433 (1997).
The drive by the semiconductor industry to further reduce solvent usage is clear from the final report of the Workshop on Pollution Prevention Research Needs for the Semiconductor and Electronic Industries and Their Suppliers.
Thus, there is a recognized need for the use of environmentally benign ceramic precursors in general and specifically for use in the fabrication of bismuth-containing ceramics for electronic applications, such as strontium bismuth tantalate/niobate materials (SBT, SBN, SBTN).